Orthogonal Frequency Division Multiple Access (OFDMA) technology is getting very popular in modern communication systems since the OFDMA technology can efficiently support multiple mobile stations with limited bandwidth and easily provide Quality of Service (QoS). The OFDMA technology is a multiple access version of orthogonal frequency-division multiplexing (OFDM). OFDM is a modulation technique for data transmission based on frequency-division multiplexing (FDM), which uses different frequency channels to transmit multiple streams of data. In OFDM systems, a wide channel is divided into multiple narrow-band subcarriers, which allow orthogonal modulated streams of data to be transmitted in parallel on the subcarriers.
In OFDMA systems, multiple subscribers can simultaneously use different subcarriers for signal transmission. Thus, in an OFDMA system, multiple data bursts can be transmitted from a base station to multiple mobile stations in the same time frame but allocated in different frequency subcarriers. Consequently, an OFDMA system can support multiple mobile stations using different subcarriers.
In OFDMA systems employing certain forward error correction (FEC) schemes, such as convolutional codes and convolutional turbo codes, the decoder performance can be improved by using soft bits (or reliability information) rather than using hard-decided bits. For instance, soft bits are used as inputs for turbo decoding and Viterbi decoding that are based on Maximum A Posteriori (MAP) and Maximum Likelihood (ML) decoding rule, respectively. Given a quadrature amplitude modulation (QAM) constellation, one commonly used metric for de-mapped soft bits is log-likelihood ratio (LLR). Implementing exact LLR is a demanding task, especially for high-QAM modulated signals. Also, when the received bits involve any scaling, e.g., by channel, in addition to added white Gaussian noise (AWGN), these effects must be taken into account when generating the soft-bits. In terms of fixed-point implementation, the required number of bits to represent soft information needs to be minimized, because in general soft bits and some of their derived metrics are stored in memory, while soft decision decoders are decoding entire sequence of bits from the received packet.
Thus, there is a need for a soft-bit de-mapping device and method of generating soft bits for decoding that reduces the required number of soft bits to represent reliability information.